Distance protection is one of various methods commonly used to recognize a fault in a power transmission network and to react on it by isolating the faulty transmission line from the rest of the power network. Distance protection is achieved by attaching distance protection relays to the ends of transmission lines and by measuring phase currents and phase voltages. From the measurements, an impedance is calculated which, in case of a fault, is representative of the distance between the measuring equipment and the fault. In the following, this distance is called distance to fault. In dependence of the value of the calculated impedance different distance zones are defined. A time limit is assigned to each distance zone specifying when the faulty line is to be tripped by the corresponding distance relays. The smaller the calculated impedance the closer is the fault to the measuring equipment and the shorter is the time limit before the line is tripped.
In EP 0 671 011 B1 a method is disclosed for determining the relative distance to fault from a measuring station located close to one of the ends of a transmission line. The measuring station provides measurements of phase currents and voltages of the transmission line. The method is based on an extended fault model which takes into account the zero sequence impedance of the line. Additionally, the method assumes that the fault current, i.e. the current through the fault resistance, is fed to the fault point from both ends of the transmission line.